text:
Do any aircraft carry spare parts for making repairs?
I was curious to know, are there (or were there) any aircraft that regularly carry spare parts to be used for replacement or repairing the aircraft itself? If so, what are those spare parts?
I'm not talking about built-in redundant systems like a second radio, but rather actual physical spare parts. For example, a car carries a spare tyre.

Carry spare part as in a cargo plane or spare part as in extra redundancy (2nd engine as example) or extra spare part as in extra engine in its hold to swap with the one on the wing?

that's why I gave an example too.. For example if every Radio communication failed, will aircraft have a satellite phone to establish communication with Ground/ATC?

Are you asking about commercial aircraft or experimental craft going to undeveloped places?

commercial. Experimental would carry some communication related stuffs along with Test Equipments (ATE) and there won't be any passengers except experts/technicians

Are you talking routine usage of the aircraft, or special charters? Routine usage, is generally a no, a special charter that's security or time sensitive, would more likely be carrying extra stuff just in case.

I didn't had any idea about routine or special charter aircraft in my mind, when I drafted this question.

Do fuses count? 'cause the FAA requires you carry spares.

I had an AF fly from São Paulo to Paris wait for the next scheduled flight from Paris to bring spares from Paris

@egid: Curious - where do modern aircraft use fuses?  I thought they used exclusively circuit breakers ('cause you can reset those in flight if need be, as well as use the breaker as a switch to cut off power to something in an emergency)...

South African Airways flights to 'less developed' locations in Africa often carry a mechanic and some spares on board. If there are no suitable repair facilities, supplies or maintenance personnel available at the destination, then their own guy can fix any minor mechanical issues. If there's a major problem then they would have to fly in additional repair supplies and staff (and fly out the stranded passengers).
I found this out on a flight from Johannesberg to Lilongwe (Malawi). A seat was broken in the passenger cabin and SAA's on-board mechanic was able to fix it after a short delay. The captain explained briefly why he was on board.
I assume that other airlines or operators who operate in undeveloped areas would do the same. It's a lot cheaper to carry a mechanic and a few spares than it is to have an aircraft, crew and passengers stranded. If anything on the MEL is broken then an airliner isn't airworthy so even a 'minor' issue can ground it.
I guess that the military might do the same for similar reasons, but I have no idea at all about that.

There was a television show about Brussels Airport on our national television station earlier this year where this was confirmed. Brussels Airline still has routine flights going to Kinshasa in the Democratic Republic of the Congo because of Belgium's history there and they take parts with them, as well as a mechanic. They actually had footage from the inside of a cargo hold of a regular Airbus or Boeing where an entire wheel was carried in case a tire blows, as well as equipment to do repairs and boxes of spare parts.

Can you add in the (approximate) date of your Johannesberg-Lilongwe flight to give us some context?

All of the 747-100/200 freighters I flew in the 1990s had FAKs (Fly Away Kits) aboard. I just checked a couple of the weight & balance data sets I still have from having done weight & balance work for one of them up until 2013, and the weights were 1967 and 1939 lbs.
If the aircraft was a nose loader, the FAK was usually along the side of the in the vicinity of the retractable ladder used to get from the main deck to the upper deck. If the aircraft was not a nose loader, the FAK was always up against the main deck forward bulkhead.
The FAKs were constructed of heavy duty plywood and opened from the top. One of the airlines had two BOW configurations for each aircraft, one with its FAK, one without, to expedite weight and balance calculations with or without its FAK.

Don't we all love those acronym-makers? FAKs, FAC(A)s...

@DeerHunter Yes, indeed, but fitting FAK into an already crowded screen for programming weight & balance calculations is easier than the full "Fly Away Kit", especially back when the screens were 80 characters across and 25 characters down, or the old HP-45 which was only 40 characters across and was a popular handheld for w&b usage and that ran DOS.

@Terry HP-95, maybe? The -45 was a scientific calculator. :)

@JamieHanrahan Good catch! It was indeed the HP-95. Another senior moment on my part. At least I got the last digit right. I should have done some Googling knowing that my memory is questionable these days. https://en.wikipedia.org/wiki/HP_95LX

BOW? What's that TLA? :)

@egid BOW = Basic Operating Weight. It's the empty weight of the airplane plus the crew and their baggage. In other words it's that weight which doesn't change from flight to flight assuming you're using standard weights for the crew and their baggage. It's that weight (and its c.g.) that you typically start with when working a weight & balance. For an example usage, go to 747.terryliittschwager.com, and choose any aircraft. The BOW is the first line in the BALANCE BOX at the upper left of the aircraft page that will come up.

I've heard what you're describing called a "Fly Away Kit" or FAK.  After some brief googling, it seems the exact contents vary depending on the aircraft, operator, and the manufacturer of the FAK, but most seem to contain tools, spare tires, and other items for simple repairs.  Smaller kits are designed to fit behind a panel in the main cabin, while some larger ones are designed to fit in a Unit Load Device in the cargo hold.
I found a brief description here.

A fly away kit is a bunch of small spare parts and some tools and
  fluids to get the plane out of a place you don't have any support for
  the airplane. We have one on all of our 727's because we operate into
  remote places that would take forever and a day to get a simple part
  to us that is a show stopper for the flight, it allows us to swap the
  part and fly away. Things like spare landing light bulbs, radar
  altimeters, instruments, gyros, gauges and things like that are
  generally what I have seen and used from in there. In addition we
  usually have a spare tire and sometimes a brake assembly somewhere in
  the belly.

Example of kit from airliners.net:

Yes. If a US-registered aircraft is to be operated at night, it must carry:

§91.205(c)(6) One spare set of fuses, or three spare fuses of each kind required, that are accessible to the pilot in flight.

Now, as far as other types of spares go - no clue! But some spares must be carried if certain requirements are met - eg, your airplane has fuses accessible from the cockpit. In the event of an electrical problem, the flight crew can replace the fuse as required.

When I flew C-130 transporters in the RAF, we always carried a FAK and a 'Ground Engineer', but then the extra weight was of little consequence compared to the delay of the mission in some remote location.
I now fly Business Jets, where there is significantly less room, less available (spare) payload weight, and certainly no-one qualified to make the repairs on board, so the only thing carried is a small set of wheel chocks, a couple of cans of oil and clamps for the trust-reversers.

Normally planes don't have major spare parts. Instead of parts they have redundancy in almost every flight system, which under certain circumstances an experienced pilot can take advantage of in order to keep operating the plane safely. Have in mind that the weight issue in a plane is very important. That's why a popular phrase between pilots says:

Every takeoff is optional, but landing is mandatory

Isn't there also a saying which goes something like "a good landing is one you can walk away from; a great landing is one where the plane can fly another time"?

Google '747 5th engine'.
The early days of the 747 program did not have spare parts readily available at remote locations.  The 747 was originally designed to externally carry a 5th engine under the left wing in the event one 747 had to ferry an engine to a grounded 747 somewhere remote.

This was used to ferry engines for other aircraft, not for use as a spare, which is what the question is really looking for. They still have to ship new engines and parts, they just use different methods now.


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In which phases of flight should the carb heat be turned on?
I have about 200 flight hours in a fuel-injected C172. I have just jointly bought a C152 with a carburetor and I am getting used to using the carb heat. I am scared whenever I hear stories about engine failures due to carb icing.
What I know is to turn CH on when:

RPM lower than 2000
When visible moisture is in the air and when it's raining

But should I leave CH on when taking off in the rain? Can you give me some ideas about other phases and conditions of the flight when I should turn on the carb heat?

You usually have some indication of carb ice before the engine shuts down, when it starts running rough turn heat on and leave it on. I would not use it on take off because of the low powered engine that the 152 has.

The short answer is "do whatever it says in your POH"; it should tell you whether to have it on or off for takeoff, for example. We already have a few related questions that might help you: here, here.

One "gotcha" that some people don't get is that turning carb heat on can, in some circumstances, make ice accretion more likely.  Air well below freezing can still have a moisture content but it's already frozen and won't stick to the venturi.  Warm it up to just above freezing and it's now very cold moisture that will freeze on the venturi.  As Pondlife says, POH.

Carburettor icing is rare at high RPMs, as there will not be much of a venturi effect. To have carburettor icing in such a scenario, it would have to be very cold and very humid/wet, in which case you would probably also encounter airframe icing, and might want to reconsider your flight.
Applying carby heat has two downsides:

The air will be unfiltered, meaning dirt or other contaminants can be sucked into the engine;
More importantly, the warm air means you won't be able to reach your usual maximum RPM.

These are two undesirable traits for takeoff.
The POH in a Cessna 172 I fly says to have carburettor heat off for takeoff and landing, but only really says it should be on when you suspect icing. I was taught to be much more conservative - prevention is better than cure. I apply carby heat whenever the RPM is below the 'green range', unless it is especially hot and dry outside. However, I remove it when I'm about 30 seconds from landing, in case I need that extra power for a go-around. You also shouldn't apply it 'half way'. Either it's all in or all out.
Boldmethod has a good article on why it's important, and you should pay attention to the conditions where it is most likely to occur.

Why is there less of a Venturi effect at high RPMs?  Higher RPM means faster airflow through the carburetor which means lower pressure which means lower temperature.  Right?

@Maxpm the throttle butterfly valve setting is what largely dictates the RPM - if you close the valve, resulting in low engine rpm, you still have more or less the same airflow hitting the throttle (prop speed probably influences that too but I digress). Anyway, the venturi effect we're referring to is that caused by the butterfly valve. When it is nearly shut but not quite, bernoulli tells us that the air will accelerate through the small gap, lowering pressure and temperature. Make sense?

Yes, thank you.

You should refer to your POH for the official word but according to this copy of the 152 POH,
For a rough engine

A gradual loss of RPM and eventual engine roughness may result from
  the formation of carburetor ice. To clear the ice, apply full throttle
  and pull the carburetor heat knob full out until the engine runs
  smoothly; then remove carburetor heat and readjust the throttle. If
  conditions require the continued use of carburetor heat in cruise
  flight, use the minimum amount of heat necessary to prevent ice from
  forming and lean the mixture slightly for smoothest engine operation

Similar notes are made in the cruise section

Carburetor ice, as evidenced by an unexplained drop in RPM, can be
  removed by application of full carburetor heat. Upon regaining the
  original RPM (with heat off), use the minimum amount of heat (by trial
  and error) to prevent ice from forming. Since the heated air causes a
  richer mixture, readjust the mixture setting when carburetor heat is
  to be used continuously in cruise flight.

And also

The use of full carburetor heat is recommended during flight in very
  heavy rain to avoid the possibility of engine stoppage due to
  excessive water ingestion. The mixture setting should be readjusted
  for smoothest operation.

for a cold start without preheat,

The procedure for starting without preheat is the same as with preheat
  except the engine should be primed an additional three strokes just
  prior to pulling the propeller through by hand. Carburetor heat should
  be applied after the engine starts. Leave the carburetor heat on until
  the engine run smoothly.

Generally you should run carb heat any time you SUSPECT carb icing. You can take a look at this AOPA brief on carb ice.
You can also check out this brief from the NTSB.

Just to add my knowledge here as a robbie pilot(R22/R44):
Carb Ice can happen any time you have temperatures between -7°C and 38°C and humidity at 50%-100% and are most likely between -7°C and 21°C and more than 80% humidity.
When you have a CAT Gauge, you apply at least enough carb heat to keep the CAT gauge happy(out of the yellow arc) PLUS any time you use less power (less than 18" MAP for my helicopter/engine) and for start up.
Nowadays the Carb Heat is going through the airfilter on our aircrafts as well.
And since our engines are derated for better altitude performance, all we do is open the throttle a bit more when we use carb heat.
So unless we are at altitude, close to our Hover ceiling/full throttle line, there are no draw backs to using carb heat.
What means to me that I will totally overuse the carb heat, even if the CAT gauge is about to burst from heat.
EDIT: According to NTSB carb icing is possible all the way down to 35% humidity. (But technically ice can form anytime there is water. So even below 35%. Never assume carb ice is impossible (IMO).

Welcome to AviationStackExchange. Having flown the R22, R44, and C152, I would highly recommend referring to the POH of the specific equipment you are flying rather than guess. Following the procedures would mean using carb heat in the C152 at low RPM settings. If you use it at higher RPM settings on or near the ground on a soft-field, you run the risk of ingesting debris through the unfiltered air. And, the C152 does not have a CAT or MAP gauge. One of the checklist items for prep to land is full or best power mixture. Adding carb heat may cause an overly rich mixture and flooding.

By the way, using carb heat reduces your engine power. This could be crucial when climbing out of ground effect on a go-around/missed approach. In a fully fueled R-22 with two people on a near 40° summer day, I would not use carb heat. Then again, I would have to ask my instructor. So far, we have never used it regardless of the humidity. And, we are not heavy guys. Also, applying partial carb heat is highly discouraged per the POH and instructors in training airplanes.

When I was taught to fly in an Aeronica Champ in 1966 my instructor would always tell me to apply carb heat just before reducing the power for landing. Leave the carb heat on until you are going to reply the power for a go around. As others have stated if roughness was detected ad carb heat. My instructor taught Sully how to fly in 1967. :-)

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text:
Is there a maximum possible size for an airplane?
Is there any theoretical or practical limit to the maximum number of passengers - and therefore size - one can build an airplane for?

hmm, approx 7Bn, as any more than that would be wasteful and unusable.

@Jamiec it took me a minute to get that... At first I thought it was a Boeing reference.

With all due respect, I don't understand the down votes or the votes to delete. It seems to me to be a perfectly legitimate question borne of curiosity from one who is not in the aviation field.

@Terry Definitely! And the first time I saw an A380 up close, my first thought was "that's crazy, how much bigger can they make these things?"

VtC because it the answer is "essentially limitless". Sure there are various structural limits, but we've surpassed the limits that were in place in the 1930s, so there's no reason to assume we won't do so again in the next 30-50 years. It seems like a very broad question with no good answer that's not opinion based. See first line of Dave's answer.

There have to be structural limits beyond which material just won't hold up, or will be too heavy to fly. There is also a limited amount of atmosphere to fly in. Seems kind of like a Worldbuilding question but I think the subject fits well here.

@mins - A) I did say "essentially", not "completely", and B) who's to say what materials will be available to us in the next 10-50 years that will allow us to overcome these issues, much as light-weight AL alloys and carbon fiber have overcome many of the limitations that were in place 30-50-100 years ago.

@FreeMan: Regardless of the materials, I think you reach a practical limit when you have to start giving your wings a downwards curve so the tips don't stick out of the atmosphere :-)

Theoretically limited by the amount of atmosphere to fly in. In practical practice, the size of an Airbus.

I would think it would be the other way around - a limit on the size, causing a limit on the number of passengers.

Setting aside the physics and engineering, there's an international agreement to limit aircraft wingspans to 80m, because of the width of runways at various major airports. The A380 is 79.8m.

Are we limiting this to specifically winged crafts?  What about lighter-than-air vehicles such as blimps or hot air balloons?  If all it has to do is float above ground and not travel (or at least not travel quickly), in theory, these could be made much larger, but it would get somewhat impractical above a certain size just in terms of materials.  Given an infinite supply of materials and a large enough atmosphere, however, I think you could make a floating vehicle of near-limitless size.  (In a spherical-cows-in-a-vacuum sort of way.)

This question is now in the top 10% of the site! Not bad for one resuscitated from the closed :-)

@mins - Now you have an inkling of what goes on at WB: absolute what if madness.

@Mazura: Right! But in 1975, engineers were ready to swear on their children head that in mythical year 2000 all planet's inhabitants would have their own helicopter. We wouldn't use it to go to the factory, as there would be no need to work, thanks to robotics advances benefiting to all. Before they had a dedicated site, WBers were already active :-)

@Terry Hi! I didn´t read your comment up above, until now, but you surely hit the nail on the middle of its head!

Related potential ASE question- would it be practical to build an aircraft so large that its fuselage and wingspan must be significantly curved to accomodate the curvature of the earth's surface?

@jamesqf there’s the clue - if an aircraft were a ring that reached around the entire planet then it wouldn’t need to generate any lift, since the bet force would be zero.  If you visualise it as a narrow thread then it would quickly distort and collapse, but with small correcting forces it could remain stable, and if it generated a small amount of lift then it could be stable even with minimal structural strength.

@Mazura Actually, if/when we design and implement sub-orbital transport, Airbus' will seem quaint.

Yes, there is an upper limit, but that upper limit might change with technological innovation.
An airplane flies because of the lift coefficient $L=\frac12\rho v^2 A C_L$, with $v$ the airspeed, which is a combination of the speed of the plane and the wind speed, $\rho \approx 1 \, \text{kg m}^{-3}$ at a theoretical mimimum of 5 km height (remember that most planes reach 10 km, but I took this a little more extreme to show an upper limit), $A$ the area, and $C_L$ an coefficent with a typical value less than 2, that might change with technological innovation.
So the only factors we can influence are $v$ and $A$. However, if we increase $A$, the mass $m$ increases faster than the area $A$ because there is more material needed to avoid the plane form breaking under the huge forces. Increasing $A$ quadratically gives more than a quadratically in $m$, and hence in the needed $L$.
If we increase $v$, we need more fuel. The amount of fuel per distance unit increases linearily in $v$, because it increases quadratically per unit of time in $v$. Hence $L$ increases quadratically where $m$ only increases linearily. This means that we might do something with increasing $v$. That means that airplanes have to go faster before liftoff, which will need drastically longer takeoff lanes.
Note that we can't keep increasing $v$ because we can't lose control.
In summary, things we can improve are $v$, the speed, $C_L$, with technological innovations and $\rho$ by lowering flying height. However, it is not practical.

I won't pretend to understand the equation, but I do recognise the square-cube law when I see it, and I think it should be explicitly named and a relevant resource linked to.

@Pharap It is not really the square-cube law. It can also be less than cubic (that was a bad assumption on my point, so I edited my answer), but it is certainly more than quadratic, because there is really extra material needed to increase strength. As Ben Crowell points out, the shape can change, as in Peter Kämpf's answer. However, we can't to that with, say, four planes, because it is likely to break in the middle, unless we add more materials and hence more mass to make it stronger.

@wythagoras I think the square-cube law is still worth a mention at least. The actual values might not be completely cubic due to shape change, but the relation to the square-cube law might make the explanation easier to understand for those less well-versed in mathematics.

I think the scaling law is not a square cube law. But if the lifting area gets twice as big, the plane certainly can´t get twice as big too, because the mass for the airplane will be more than twice as big.

The size and shape of aircraft often are decided by other things besides efficiency. For instance, the Super Guppy, which was designed to carry ungainly loads like rocket boosters. Someone I once knew flew on an empty Super Guppy and played touch football while the plane was in flight. http://d2rormqr1qwzpz.cloudfront.net/photos/2015/07/08/78070-flight_b.jpg

I am not sure if effects other than linear apply in a well designed system. The formation of say 2 planes flying together has the same ratios and fuel consumption per mass as a single plane. Just connecting them on wingtips should not worsen the things dramatically.

@h22 You seem to be assuming that the planes are flying in a straight line through uniform air. As soon as the plane wants to bank to initiate a turn then there will be significant stresses through your wingtip to wingtip connection.

It is no accident that the biggest birds are flightless. The ability to fly goes down with increasing size, so there is also an upper limit for aircraft. The main reason is that as size increases, masses go up with the cube of the size increase while the load-carrying structures like wing spar cross sections only grow with the square of the size increase. This power law is the simplest of the scaling laws.
Since the loads on an aircraft's wing depend not only on its size, but also on many more parameters (angle of attack, speed, aspect ratio …), there is no clear boundary, and advances in materials help to shift the size limit up. If one would try to build the biggest airplane in the world, the wingspan could easily be double of what todays biggest airplanes measure, but the utility of this airplane would be very limited.
Using this airplane for passenger travel would add more restrictions like the number of emergency exits and the maximum distance to the nearest exit, but this could be overcome by using several smaller fuselages. A double-hull airplane would also spread out the payload weight, so the wing would experience a reduced root bending moment. Going from this (source):

to this (source):

would immediately raise the size limit substantially. However, it would require new, wider runways to take off from. And adding more fuselages to the same wing will run into flutter problems soon.
The next indication could be designs which have been studied and deemed feasible but were eventually not built for economical reasons. Here the biggest are ground effect vehicles: Flying slowly in dense air pushes the size limit up. The Boeing Pelican was planned with 152 m wingspan, and Beriev proposed one with 2500 t take-off mass and 125.5 m wing span.
I would guess that a wingspan of 200 m is still feasible, and when spreading the weight even 500 m should be realistic, but totally unpractical. Taking a lesson from history, this would be a multi-hulled flying boat which flies in ground effect, similar to the (mono-hulled) biggest aircraft record holders in the 1920s.

As your second picture shows, there's theoretically no limit: you can keep tacking wings, engines, and fuselages on forever.  However, as the crashes of experimental high-aspect-ratio aircraft have shown, keeping everything coordinated is a bit tricky, as your airplane no longer acts like a rigid structure.

@Mark When your plane is so big you have to take into account the curvature of the earth from wing tip to wing tip, you're in a whole new realm of engineering.

@corsiKa True, but it is still a realm of engineering, rather than physics. From a physics standpoint, it's possible. From an engineering standpoint, it's completely impractical. Also, I think curvature of the Earth is the least of your problems if you're trying to build the aircraft described in Dave's answer.

@reirab But then you have the limit impressed by going full circle around the earth  Maybe you could go around a few times, like a corkscrew -- but at some point, you'll fill all the gaps and can't go around any more. So I guess the theoretical limit of an airplane is a Dyson Sphere? :)

@yshavit Once you go all the way around Earth, it's not really an airplane anymore. Even that isn't the theoretical limit to the size of an airplane, though, as there are planets much larger than Earth which have atmospheres.

@PeterKampf -- I agree, to a point. "It is no accident that the biggest birds are flightless." Sure, but there's an evolutionary part of this. It's not simply that they've "gotten bigger" over time -- they got bigger, because the size offered them something that allowed them to better adapt to their surroundings. Airplanes do not have this "environmental pressure", so I think the line is starting to blur with this analogy. With this in hand, I completely agree with the rest of your statement! Great examples.

Sounds like it would be theoretically possible to make a very wide airplane by using multiple fuselages side by side. Would it also be possible to to make an airplane with a single very long fuselage, and several pairs of wings?

@TannerSwett: Adding fuselages sideways will run into flutter soon. There is definitely a limit. Adding wings lengthwise will make the whole thing inefficient - lift is produced by accelerating air downwards, and if this is repeated with the same air, the rear wings will add little lift but lots of drag.

Curvature of the earth over 1000 m is 0.08m (8cm), I'd imagine when this becomes an issue, it won't present a problem.

@PeterKämpf Well, apparently nothing that cannot be kept under control even by an hobbyist (https://youtu.be/mrJgoQz5wSU).  Let alone some sophisticated computerized controls, like those used to fly aerodynamically unstable military planes, or 737 MAXs (ah well).

@Peter-ReinstateMonica: Great video; thank you for sharing the link! Best quote: "It's more oscillatory than I hoped for" - just as I predicted. But damping is working in their favour and putting hinges between the wings was smart.

Theoretically, unlimited (well bigger than is practically necessary)...
TL;DR
Airplanes scale fairly well and it would be physically possible to build and airplane of just about any size. Granted there are some things that come into play from a structure standpoint but there are surely ways around that. You may have to stray away from the traditional single fuselage, two wing and empennage design but none the less.
There are lots of realistic factors that will stand in your way before you even need to design such a plane.

You don't have enough money to build such a plane
Boeing does not have enough money to build such a plane
Juan Trippe has no interest in such a plane so there is probably no reason to build it.
There are no runways that could handle such an aircraft. Planes like the A380 and 747 are already route limited by runway length/weight capacity. You would need to modify runways to handle anything significantly bigger. That is of course assuming that it lands in a similar fashion to most airliners (i.e. not VTOL)
What route is it going to fly? Planes are not the size they are because we cant build them bigger, they are the size they are because the routes dictate them being such a size. Do you really need to move 1,000 people on a given route at once? How many routes have this much travel density?

Let's look at this hypothetically, Jamiec makes an excellent point that a plane that has the capacity in excess of 7Bn would be kind of useless so let's take that as a maximum. XKCD what if covers this in a similar question and estimates that shoulder to shoulder all the people on earth take up roughly the size of Rhode Island. For the sake of argument lets say that you would need seats and toilets and what not for this many people so to fit everyone on earth in a plane you would need maybe twice the size of Rhode island or a bit more. Unlike that question we can build up so a 4-8 story (or any amount) of plane would be plausible. The average FAA person weighs about 180 lb (81.65 kg) so you would need to lift
1,260,000,000,000 lb Or 630,000,000 tons (572,000,000,000 kg or 572,000,000 metric ton )
To provide a frame of reference the A380 has a maximum structural payload of 330,300 lb (149,822 kg). Keep in mind this is only payload, you also need to lift the weight of the airframe, engines, and fuel (if you actually want to go anywhere). So basically you would need a multi level plane close to the size of Rhode Island that had most of the combined thrust available on the planet and enough liquor to keep everyone calm for the flight.
The structural issue in some regards boils down to wing loading. One commonly self imposed limit these days is that most planes are typical low wing cantilever monoplanes so we view things in relation to that. In other words the fuselage may stress about the wing mounting points and the wings are generally long and low but there is nothing preventing us from using an alternate design with multiple wings or a full lifting body design to accomplish the structure we need. As such the idea of big wings can be overcome and historically this is how the problem was solved in early aviation (tri-planes, etc...) The weight issue (from a practical standpoint) is something we concern ourselves with for efficiency reasons. If we are just building a giant plane we can use jets, rockets and all manners of high thrust devices for the purpose of science. You could fly a cement plane if it was shaped right and you had enough thrust.
Remember if Thrust > Drag and Lift > Weight you will fly (I learned this the first day in flight school). Doing so in a controlled and organized manner took significantly more time to learn....
-- Edit --
Since the question has been edited to involve the passenger count there are other issue that crop up.

You need to realistically load and unload the plane which takes time. A plane that takes too long to load and unload will not be economical to operate.
Weight (Americans are getting fat) (see above)
You need a reason to move that many people that far all at once.
Depending on flight length you need to consider food, water, and restrooms to accommodate everyone (waste tanks are not infinitely large).
Are there even enough people in one place? Planes move people (and often stuff) from one place to another but let's take NYC for example which has a population of about 8.5 million people the chances that all of them are going to the same place at once is near 0%. So you don't need an 8.5 million passenger plane, but you can start a bit smaller.

From an airline standpoint (The people actually buying) planes like the 747 are already big enough and expensive enough. The 747 nearly bankrupt Boeing at the time but has had a fairly successful run since. The A380 is fairly new but it will be interesting to see how it changes the game.

We could always go back to really big flying boats :-)

To save time Googling for those unaware of Juan Trippe, he was the founder and CEO of Pan American World Airways, and he wanted something bigger than the 707. He famously told Bill Allen of Boeing concerning the proposal to build the 747, "If you build it, I'll buy it". Allen replied, "If you buy it, I'll build it."

@Terry is exactly correct, I have also added a link to clarify a bit on who he is.

So, since the people take up the size of RI, the plane would take up the size of at least MA in wingspan.

Point 4: Seaplanes could (theoretically) use "zero-construction-cost" runways much bigger than land-based airports - but the logistics of getting the self-loading cargo on and off the plane becomes more problematical.

The XKCD What If answer that you cite addresses your point 3. We need a plane that size to get everyone out of Rhode Island after they all find themselves standing there shoulder-to-shoulder.

Concerning waste tanks: I guess as your plane size approaches the Rhode Island size, its total waste tank size may approach zero, since the contents could just be vented out without the problem of affecting  non-passengers... 3:-)

Would be also interesting to consider taking the passengers on the plane. How much time would it take to bring everyone on board (the plane should be built in China probably to decrease costs of bringing people to the airport).

The first bottleneck is a SECURITY check time, we already experience it, and it needs to be solved first. Key observation -> "A plane that takes to long to load and unload will not be economical"

Why is TLDR the second sentence of this answer?  Is it a signal to stop reading?

Certainly a plane can't be bigger than Earth. I would even say 10 miles long aircraft is quite useless, because you need to have some transportation inside of it to deliver all passengers on theirs seats.  An airport for keeping such places should be also a big. Therefore limitations are coming not from weight/lifting power, they are coming from practical uses of such big planes.

It sounds crazy, but what about an airplane that has the form of a circle and circumvents the Earth?

@descheleschilder That's called a Dyson Ring. You'll want Space Exploration Stack Exchange or World Building Stack Exchange for more information on those.

There is nothing in the question which limits the plane to flying over an Earth-sized planet. A plane could designed and intended for flight over a much larger planet (e.g. Jupiter).

@Makyen, I doubt Jupiter's atmosphere could be described as "air", so airplane might then not be the exactly correct term :-) Oh boy, now we are getting into astronomy: what size planets could sustain an air-like atmosphere....

And of course on Jupiter, there's nowhere to land.

@descheleschilder At that point, can it really be called an aeroplane? Circumventing the earth would make it impossible to land, thus it would either need constant fuel or it would have to orbit the earth (outside the atmosphere). It would also have to be very carefully planned as the Earth isn't spherical. As corsiKa said, that's a question best asked at World Building, since this is getting into the realms of speculation and fantasy.

@Pharap I can´t respond to that other than that you´re completely right!

@Pharap The plane can land if it doesn´t circumvent the equator. Of course, a very big landing strip should be made, wich is practically impossible to do. Will a scaling law prohibit this?

Maybe we can use a smaller aeroplane to deliver the passengers to their seats. physics.se: can an aeroplane fly inside another aeroplane?

The Convair XC-99, cargo and passenger version of B-36 bomber, was rejected by airlines, they say, because airports were not prepared to handle over 200 passengers and their luggage disembarking at the same time, one of the main problems in the giant Airbus may be that it's close in undercarriage width to some airstrips, you remind an aircraft carrier ship deck take off and landing when watching an Airbus 380.
The type of take-off and landing of Saab Viggen, designed to eventually operate from Swedish highways, is not acceptable for commercial flights, and the marginal vortex in the tips of Airbus 380 is so powerful, that the airport must be closed for some minutes after one of these giants used it, in order to avoid an smaller airplane being send down when entering the turbulence, thus limiting the whale airplane advantage in passengers per day in this airport.
Regarding huge size flying machines, you may have to look at Sci-Fi and UFO writers, eg., 'Rendezvous with Rama', or the case of a commercial flight pilot, if I remember well, going from Barcelona to Pamplona, in Spain, who watched an stationary round cloud hovering high over a dam lake, that draw his attention, and reported requesting permit from air traffic control tower to turn 360º around the cloud. He concluded the cloud wasn't a cloud, but a metallic object one and a half kilometers in size (sorry, I don't know if it was diameter or circumference), this is really much bigger than the Kalinin K-7, a heavy bomber built in 1933, that crashed for an structural failure of because a mid-air collision with a smaller airplane.
Who knows what the future may be in airplane size?
In the end, as in the movie: 'The yellow Rolls-Royce', 'Tomorrow never comes'

A flying city, such as those proposed by Georgii Krutikov.
As most of the problems seem related to taking off and landing, best probably would be to keep such a giant plane constantly airborne, very high where the air is more stable and compose of multiple modules that could take off self-dependently and then join that bigger "flying castle" - like a space station. Docking seems tricky but should be possible as areal refueling is possible.
This castle could be solar-powered or nuclear-powered, for instance.
It is more difficult to think what would be the use of it.

A larger aircraft could exist as basically 2 aircraft glued loosely together side by side with flexible joint and computers to carefully manage engines and controls to keep all the pieces from breaking apart...   you could extend it by making entire aircraft a flexible wing that follows curve of earth.
A larger aircraft could be mostly a "lighter than air" airship that is a little heavier than air and using "lifting body" forward motion to get the extra little bit of lift needed to rise.
"Limits" can be overcome but end result becomes less and less practical beyond a certain size.

In conventional world the practical size is dictated by typical airport runways.

There is no theoretical limit, given a large enough planet with a large enough population. But there are several practical limits.
Weight scaling
Most significant, and badly misunderstood by several answers here, is the issue of weight. Where a bird is a solid object and scales with the cube of its span, an aircraft is a hollow shell, a surface, and so it actually scales with (nominally) the square of its span. This has important implications.

Ignoring Reynolds number, a wing profile can only support the same loading (weight per unit area) at any scale. Double the dimensions mean four times the area so four times the lifting capacity.
With the same wing loading, the stressed skin need be no thicker. So it only weighs four times more.
An I-spar doubles in depth. Strength of a given amount of material increases with the square of its depth, so it can carry four times the stress, which accounts for the aircraft weight, for the same amount of material. Except, the spar is now twice as long. The stress is also proportional to that, so it doubles. So you need twice the cross-section, twice as long, making the spar four times the weight in all.

So theoretically the size can be increased indefinitely, with the number of passengers increasing with the square of the wingspan. But eventually, practical limitations kick in.
One problem is that the skin needs more stiffeners (ribs and subsidiary spars) to stop it wrinkling. These must themselves be braced by more cross-pieces. These cross-pieces must be stiffer to cope with being twice as long. The web of an I-spar (the vertical bit) is an example. Designers cope by keeping the various members spindly and adding more subsidiary stiffeners to hold the main ones in position, a technique known as increasing cellularity - everything inside is a skeletal fabrication, there are few solid sheets any more.
But the practicalities of manufacturing and materials limits how far you can go with all this, and it is expensive. Unless you are very crafty, you soon reach a point where adding another level of stiffeners also adds another level of weight. So at some point the design can go no bigger, and weight starts to creep up as corners are cut (or, rather, filled in!) to ease manufacture and costs.
Control surfaces
There is also a problem with the control authority and responsiveness of large aircraft. An aileron four times the area will exert four times the control force. The wing has four times the weight, so it accelerates at the same rate. But it also has twice as far to move in order to reach the same bank angle, so it takes longer to get there. If no control system changes are made, the plane will respond sluggishly and pilots must make decisions early. This can be dangerous if an instant flight correction is required.
So large aircraft have extra flight control surfaces added, such as wing spoilers, inner flaperons, larger tail surfaces and the like. Clearly, there is a limit to how far one can go in chunking out ever more, ever bigger pieces of an ever wider-span wing, while keeping it structurally stiff. Moreover the bigger control surfaces suffer the same problem with sluggish response times.
Systems creep
All this brings further issues over the amount of control, monitoring and related equipment carried in order to fly the plane. Landing wheels tend to become more numerous rather than just larger. Complex and massive control surfaces and landing gear need complex and massive control systems, bigger payload space tempts the designer to a more complex interior fit, cubing the air can cube the provision of fire suppression, ventilation and conditioning, and so on. In practice, large aircraft scale rather more than the square of the span, though far less than the cube.
Operations
Next, there is the problem of where to take off and land. The chief designer of the Airbus A380 has explained that it has winglets mainly because longer wing tips would not have fitted modern airports. The Stratolaunch Roc has the largest span of any plane to have flown, and only a handful of US air bases are large enough to take it. Howard Hughes' Spruce Goose was a seaplane so all it needed was open water - but that's an awful lot of water to check for floating logs and other debris. So you have to find somewhere that other people will still help you do stuff on such a grand scale.
And finally there is the marketplace. Even before Covid the 747 had ceased production and the A380 was winding down. Airlines were buying smaller planes because customers wanted more frequent services on more routes. Stratolaunch have resorted to building their own payload, a spaceplane. You would have to know where your payload was coming from too.

This sounds convincing indeed! I was thinking along similar lines. Especially the weight not scaling with volume but with the surface.

The limits you are asking about have been pushed by the antonov. Anything bigger than that is certainly possible but not very much economical and may not be safe. While the global economy is in melting ice and a few solid patches here and there it would be a very unsuitable experiment to justify. Rather diverting the funds to improving the current airplanes may have a better result compared to an increase in size.

Welcome to aviation.se. While your considerations are correct, I feel that your answer lacks in detail. I encourage you in editing it and expanding it.

The first two sentences discuss the question, the other sentences are less relevant.

Amazing how a potential development can be discussed without highlighting the economic impact. It remains relevant even when some do not want to accept it.

Economic reasons are undoubtedly important but the question didn't ask about economic limits, it asked "Is there any theoretical or practical limit" which suggests the question is about engineering limits more than political, social, legislative, economic or other types of limit.

Just for clarity, my question is purely out of curiosity, but I surely don´t want airplanes actually getting bigger and bigger or that there will be more and more built. But that´s another topic.

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Is an RT license required for EASA PPL holders?
Is an RT (radiotelephony) license required in the EU for an EASA PPL holder acting as PIC in an EU-based aircraft?

EASA itself does not require a RT license (RT is usually legislated on national level), however, it gives the state issuing your license the possibility to include RT privileges as part of your license (item XIII in the license).
This is also the most common case, ie, your PPL will most likely include RT privileges, so you will not need an RT license in addition to this.
What EASA state are you planning to have your PPL issued in?

unless things changed, the Netherlands definitely requires a separate RT license. Then again, anything to make a quick Euro, they used to require one even for cellphones in the pre-GSM days...

I am still confused with it: there is separate "communication" topic in theory course and exams, we pass ICAO English as international aviation language, but, as I understand, almost all EU countries still require that license, and most of them require separate exam (for example, RTOL in England or RT in Czech) in its own language...

During your training, you will need a student license. Typically, the student license does not include radio privileges, thus the need for a separate radio license (which then typically becomes "embedded" into your PPL).

@Waked I believe radio "privileges" don't come with PPL (at least in most of the EU countries, I know 7 where it is needed for sure). Can you specify where not? Usually a separate license is required. And in some countries an ICAO language exam is needed to fly abroad

@GabrielVince Could you please list states where it is required if you already have filled "RT privileges" section in the PPL? I know only about UK and NL.

Sure SK, CZ, NL and BE. You will need a separate radio license. I really doubt there is RT mentioned in PPL though format of these documents has changed last years. I could do the exam in BE in English with no problems

@GabrielVince I don't disprove of what you are saying, but please provide references to to current regulations. As indicated by my answer, national legislation on radio DO exist and may differ between countries. The only EASA requirement is is FCL.055 (requirement for language proficiency endorsment). (Sidenote: The PPL skill test content [AMC FCL.235] includes R/T procedures). (Another interesting sidenote: Airline pilots may overfly 20 different european countries in a single day, each country with individual national legislation).

@Waked: this is an example of the regulation: http://www.laa.sk/obmedzene-osvedcenie-radiotelefonistu-leteckej-pohyblilvej-sluzby-ii/ FCL.055 only claims the pilot must prove sufficient level of English to use the radio (it doesn't automatically grant any right to do so). And indeed, you can fly over multiple coutries where all have "harmonized" legislation and mutually acknowledge all official documents (PPL, medical certificate, radio license). Let's try to fly outside EU and you will have to check internation agereements or local legislation. It is similar to the driving liecenses

@avtomaton why you insist so heavily on having RT granted by PPL? Did you get forbidden to use radio? If you can do PPL, the RT license will be piece of cake

@GabrielVince I do not insist, I am just curious. I also do not see anything wrong or difficult in that exam, but I just asked whether CAA in some EU countries approves RT privileges granted by RT section of PPL (if it exists there, of course: I do not know exact PPL forms in all EU countries), and what do pilots recently passed exams in any state have in their PPLs. Your link to SK website seems related to gliders, hang-gliders and ultralights (that kind of license definitely does not have RT-related section and the pilot definitely should have separate license to operate on-board radio).

To be consistent between posts, this is answer from another chat http://chat.stackexchange.com/transcript/message/37321336#37321336 in CZ you need VFL or OFL license (as described https://www.ctu.cz/druhy-prukazu)


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What are the spiky things on the back of the Antonov-225 vertical stabilizers?
On the Antonov-225's split vertical stabilizers, there is a 'spiked' structure pointing directly rearwards (as you can see below). What is the purpose of these two structures?

source

Possible duplicate of What is the purpose of the sharp pods under airliner wings?

@GregHewgill, I don't think the purpose is the same. It clearly has aerodynamic reason, but at that point there are only the stabilizers and they are ending rather smoothly due to the sweep, so they shouldn't need shock body. I'd rather guess it has something to do with interference between the horizontal and vertical stabilizers.

In either case, this is definitely not a duplicate, because even if the purpose was the same, it is absolutely not obvious and therefore needs a separate answer.

@mins Thank you! Yes, I will amend the post with the link.

@JanHudec: Thanks, I've retracted my duplicate close vote.

It appears to be the extension of the aerodynamic fairing between the vertical and horizontal tails (note that the horizontal tail extends outside the vertical one). It can be seen more clearly in the following image.

Image from diecastaircraftforum.com. Original image appears to be from airliners.net
This type of fairings (usually called bullet fairings) are found in 'T' tails and other types of tails (like cruciform tail) where two surface intersect each other at right angles and are used to reduce interference drag (and flow seperation). For example, the following image shows the fairing in C-141 Starlifter, an aircraft with 'T' tail.

airplanesofthepast.com

I thought about this as well, but I wonder why they extend so far backwards past the intersection?


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Carrier frequency and transmit power of a commercial aircraft
I have a few questions on the radio comms. aspects of commercial aircrafts.
At what (central) frequency does an aircraft communicate with a control tower? Is the communication over one frequency only? How much is the transmit power of the antenna per channel on both uplink and downlink? Is it per hertz?
Also please correct me if I’m wrong: From my readings, aircraft-airport communications is analog only, correct?

This seems very similar to your previous question

Just to be clear, in the elevated structure that enables those in it to see over, usually, the rest of the airport buildings, there are, typically, two ATC positions manned by different people: the 'tower controller' and the 'ground controller', and they use different frequencies. At larger airports there might also be discrete frequencies for 'clearance delivery' and 'ramp control' as well as others. When you speak of the 'tower' are you referring to the 'tower controller' or the aggregate of the controllers in the physical structure known as the tower?

In the US, non-military Aviation uses VHF frequencies from 118.00 MHz to 136.990 MHz in both 8.33kHz and 25 kHz spacing.
Generally, one radio is used for one frequency at a time. With 2 pilots, 2 radios on 2 different frequencies can be used together; for example one listening/talking to approach, or a control tower, while the 2nd listens to ATIS/ASOS for airport information.
Perhaps the big iron can do 2 frequencies at once in 1 radio unit.

Could you please give a reference to that?

Sure, Avidyne IFD540 Pilot Guide, page 1-17 for the frequencies. https://www.avidyne.com/files/downloads/600-00300-001.pdf  PS Engineering Intercom Pilot Guide Page 5, Split Mode  http://www.ps-engineering.com/docs/PMA6000_PG.pdf

Do you also have a reference showing how power is distributed on those channels?

No.  I would assume evenly.

Many military aircraft have the ability to use the "civilian" VHF band as well as the "military-only" UHF band 225–400 MHz. Pretty much all transport-category military aircraft will be on the VHF band while talking to civilian ATC, and even many of the fighter/attack aircraft have VHF capability.

At what (central) frequency does an aircraft communicate with a
  control tower?

This is different for every tower, although due to the limited discrete frequencies around the globe they are repeated, generally far enough apart its not an issue. Specific frequencies can be found for US airports on either charts or in the AFD.

Is the communication over one frequency only?

Generally every tower only has a single frequency, there are some airports or enough traffic to warrant multiple frequencies. They are often segmented by approach direction or runway.

How much is the transmit power of the antenna? Is it per hertz?

Transmit power may vary based on airport but should be constant across the frequencies.

Also please correct me if I’m wrong: From my readings,
  aircraft-airport communications is analog only, correct?

Not analog only, digital comms are used. For the airport environment there are systems such as CPDLC for delivery clearances and D-ATIS for digital airport information.

Could you please give a typical range of transmit power? Also, if the transmission is over one frequency, then why did you say ‘channels’? Cause each channel correspond to a different frequency to me.

Many towers will transmit simultaneously on a VHF and a UHF frequency. Some military aircraft only have UHF radios, so when the controller keys the mic, he's transmitting on both radios at once, so everybody tuned to the appropriate tower frequency (on either band) will hear him.

Is the aircraft transmit power the same as the tower?

@Teodorism it depends on what the airplane is equipped with, the plane I fly has a 10W radio while a 737 has a 400W HF and 25W VHF radio

Is 400 W per channel? Or total transmit power? Also, hpw many channels does a 737 use?

@Teodorism these would be good topics for new questions.

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How does air traffic control communicate with an aircraft?
I have a question on voice/data communication between air traffic control and an aircraft.
I read somewhere that control communicates with an aircraft directly, through a direct radio link, while it's within the line-of-sight. After that control communicates through satellite.
My questions are:

Why doesn't control use satellite throughout the flight?
Are direct radio links used only for voice services? or are they used for data as well?

For 1, the problem is about who pays for the satellite communication and the equipment. As the answer is nobody wants to ask the passengers to pay, then they use good old ground transmitters. But transponder broadcast is going to be relayed by satellite soon, to track aircraft over the globe (10 new Iridium satellites have been launched in orbit this afternoon by SpaceX)

@mins Thanks. Are ground transmitters located along the travel path of the aircraft? or is there only one based at the airport that adjusts its transmit power as the aircraft moves close to or away from the airport? Aren't there security issues or power safety issues involved in ground to air transmissions?

@mins: Re "nobody wants to ask the passengers to pay", you're perhaps overlooking the large number of non-commercial aircraft out there.  Making passengers (or people using air freight) pay for equipment on commercial flights would give you two different, largely incompatible, systems, with all the possibilities for confusion that implies.

"Are ground transmitters located along the travel path of the aircraft", see how the different types of ground control stations work together.

RADAR has nothing to do with communication. Communication is done using radio. Satellite communication is in no way normal.

@jamesqf: There is a need for a big change in ATM, both in coverage , digitalization and automation, as traffic is increasing. ESA is currently paving the way for satellite (Immarsat) assisted ATM with Iris, like Aireon/Thales are now deploying the global Iridium based ADS-B. Backward compatibility is a requirement taken into account, but the change has to be disruptive. Current system is at its limits, and not far from being archaic. Cf. AF447/MH370.

Is there a lag on the satellite comms?  Imagine a controller instructing an aircraft to go-around on short final and the flight crew not getting the message for a few seconds

@Teodorism if you are interested, you can listen to ATC chatter at www.liveatc.net

"Is there a lag on the satellite comms?" If the satellite is in low orbit (300-600 km), there is a lag equivalent to the one between an interstate land line. But ATC managing landings will not use a satellite to talk to an aircraft at 0.5 NM. Would you use this solution if you were a communication engineer in charge of designing such system?

@mins: Why do you think ATC is the problem?  The system works quite well as it is.  If increasing  air traffic is causing problems, they seem to be related to needed spacing (which is often governed by physical things like wake turbulence) and available landing slots at busy airports/times, rather than any defficiency in communication.

@jamesqf: The lack of communication and tracking over e.g. oceans leads to considerable problems, as demonstrated by the effort and money invested to search for AF447/MH370, not mentioning the humans aspects of being in the dark for weeks. Then in busy areas, adding UAV or aircraft (traffic is expected to double in next decade) without permanent links and automation will lead to the ATM incapacity to absorb the traffic and keeping wait time, noise and pollution at the desired levels.

@ mins: Over the oceans (and above 18K ft) is not really what I (and I expect most of us) think of as ATC.  That's more like a few tens of miles/km from the runway.  Communication there is an entirely separate problem, and one that's pretty much irrelevant to those of us who fly Pipers, Cessnas, and so on.

1) Why doesn't the tower use satellite throughout the flight?

The narrow answer is that the control tower is responsible for a small geographic area.  The "tower" doesn't want to talk to the craft at other points in the flight, so the need is not there.  It only wants to talk to the plane when it is close to the tower's airspace.
Other components of air traffic control that do need to talk with airplanes in flight also divide up regions of the airspace and have radio equipment situated to talk to the necessary craft.
A larger answer is that ground control and aircraft communications were under major development for decades before satellite communications were available.  The system we use today is heavily based on line-of-sight VHF communication radio over most populated areas.  Since these systems are already installed in many thousands of aircraft, changing to something different like satellite communication requires a significant benefit, someone to pay for it, and a lot of time to allow for the change.

2) Does a control tower only send voice services through the direct
  link? or does it send data as well?

Data is beginning to be sent via CPDLC.  You can search for CPDLC on this site and find several questions and answers about it, but it is considered only a supplement to the voice communication of ATC today.

Aren't there security issues or power safety issues involved in ground
  to air transmissions?

Unauthorized transmissions are possible, but they are managed and haven't traditionally been a big problem.  There's another question about What prevents someone from hijacking a radio frequency mid-air?.  But much of the answer is that since ATC is monitoring the frequency, unauthorized transmissions would be heard and responded to.  I'm sure that future digital delivery will change some of these issues, but that's likely to be true whether the message is transmitted via local radio transmitters or satellite.
Not sure if that's what you meant by security issues.  I'm not familiar with power safety issues.

We smaller private planes, the 2-4 seat Pipers, Cessnas, etc. are usually flying lower (except by the Rockies) below 12-14000 feet. Above that special needs come into consideration for Oxygen as the planes are not pressurized (after some time duration above one altitude, and full time above another).  We would generally talk to 3 layers of ATC - the ground/tower layer near an airport, then Approach as we start climbing up, and finally Center as we depart the airport area and start cross country at higher altitudes (otherwise, bouncing from one Approach to the next if lower down). I've never been above 12,500 myself, crossing the Rockies and talking to Kasper, WY area ATC (with mountains at 11,500! Nice day for it anyway). I do a lot of east coast region flying, and Approach and Center are the main folks I talk to via VHF radio.


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What is the speed and inclination angle of a commercial plane during descent?
I have a question based on flight track logs on https://flightaware.com.
In one of the logs I'm looking at, it says that at time 08:16:15 PM the altitude was 36,000 ft and plane speed was at 473 mph. At time 08:16:58 PM, the plane descended to 35,400 ft.
I did some rough calculations and realized that the speed 473 cannot be along the vertical plane. Then, what is this speed? Is it along the descent path? Also, what angle do planes make with the vertical plane when descending? Where can I get this information?

Why would you even think that is vertical speed?  (Assuming that flightaware is accurate in both time and altitude) A difference of 600 feet in 43 seconds works out to a descent rate of 850 fpm, which is completely normal.

Normal takeoff pitch (“longitudinal axis against horizon”) angle is, as a rule of thumb, about 15 degrees; normal takeoff flight path angle („height/distance“) maybe 10 degrees (note: an aircraft rarely points where it goes in the vertical plane; most aircraft - unless there is very strong wind - usually point above where they go). After takeoff, climbing to cruise altitude, both angles will be lower. For descent, it’s about 0-3 degrees pitch, and -3 degrees flight path angle. Every single flight will deviate from the above, but as an average order of magnitude, this should work quite well.

473 would be the horizontal speed.
Pitch angle is limited to 30 degrees in the US per FAR,
https://www.faa.gov/regulations_policies/faa_regulations/
More than that the flight is considered Aerobatic and parachutes are required.

Then, the vertical speed must be
(36,000 - 35,400)ft / (08:16:15 PM - 08:16:58 PM) sec = 837 ft/min. Isn't this too small?

600 ft in 43 seconds? Seems reasonable.  I descend in my small 4 seat plane at 500 ft/min, just to avoid too much ear popping. Jets are more or less constant 8,000 foot air pressure in the cabin, can do higher rates without too much discomfort for the passengers.

Isn't that too small for what, exactly?  Descending from 36,000 ft would take approximately 30-35 minutes.  That is a very normal descent.  You can hear and  feel the engines throttle back right when you hear "This is the captain: we've begun our initial descent, and will be on the ground in about 35 minutes"

Rather than linking to the entire set of FAA regulations, can you link to the specific one that you've referred to? (I assume 91.307?)

30 degrees up is very rarely reached on a normal commercial flight, 30 degrees pitch down virtually never.

Yes, 91.307(c) for passenger planes:  "(c) Unless each occupant of the aircraft is wearing an approved parachute, no pilot of a civil aircraft carrying any person (other than a crewmember) may execute any intentional maneuver that exceeds—
(1) A bank of 60 degrees relative to the horizon; or
(2) A nose-up or nose-down attitude of 30 degrees relative to the horizon."  And then exceptions follow for training purposes:

"(d) Paragraph (c) of this section does not apply to—
(1) Flight tests for pilot certification or rating; or
(2) Spins and other flight maneuvers required by the regulations for any certificate or rating when given by—
(i) A certificated flight instructor; or
(ii) An airline transport pilot instructing in accordance with §61.67 of this chapter."


text:
How to find the flight paths of an airport?
How can I find the flight paths near an airport (e.g., Heathrow or Pearson) including cruising altitude, descending speed (variations)?
Specifically, I need a figure showing the height vs. speed of a plane as it's landing starting from the cruising altitude.

http://www.uvairlines.com/admin/resources/EGLL.pdf

You might consider using a website such as Flightradar24, which will show you real-world realtime flight paths (including vectors or holds that may be assigned by air traffic control, as opposed to ideal approaches). You can click on a particular aircraft to see its track, then click "speed & altitude graph" on the detail pane to see speed and altitude at one-minute intervals.
There's also a speed/altitude graph for each flight tracked on FlightAware.
If you check out a variety of flights into the same airport, you can see what approach paths are used by aircraft arriving from different directions for the current wind conditions.

You can find a great deal of approach path information here but that will only give you the path, and perhaps some intercept altitudes for a given approach it will not give you any speed information. Cruising altitudes will also vary heavily by aircraft type.

Specifically, I need a figure showing the height vs. speed of a plane
  as it's landing starting from the cruising altitude.

This is broadly specific to each airframe as well as the loading of any given aircraft at a given time. But you can find the information if you know the airframe in question, there are also some limits depending on where you are in the world. The FAA (and presumably most legal entities) cant have different rules for everyone so they broadly break down approach speeds into categories. That should give you a fair idea of the approach speeds. Keep in mind these are airspeeds and local wind will alter ground speeds on an individual day basis.
If you start from the cruising altitude there will be a few phases of the decent that will see various speeds. This depends on what ATC clears you for and what the local conditions call for more on that here.

The SID, STAR and approach charts for Heathrow are here: http://www.nats-uk.ead-it.com/public/index.php%3Foption=com_content&task=blogcategory&id=94&Itemid=143.html   and that site will give you similar information for other UK airports

Look at the Approach Charts for the airport of interest.  In the US, speeds are also limited to 250 knots below a certain altitude and I think distance from an airport (I never had to worry about that, my Vne (redline airspeed) doesn't go that high!).
I don't think you'll find speeds from cruising altitude as speeds are adjusted to match traffic flow.  And cruising speed varies by airplane type as well.


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What is the cooling air exhaust velocity for the piston engine of a propeller driven aircraft?
Piston engines of propeller driven aircrafts need to be cooled. There are engine cowlings which accomodate the inflow of air to "wash" over the engines within them and which is then exhausted out into the freestream.
The model in question here would be a single block engine in the cowling of a tracktor configuration propeller aircraft. The airflow enters the cowling, washes over the engine, and then exits into the freestream.
The question is:
What would be a good estimate for the velocity of this air stream exhausted into the freestream?

I don’t understand your question. Can you please explain what you mean by “cooling exhaust velocity”.

Agree. It’s completely confusing. Are you asking “what is the velocity of the air that cools the external surfaces of the engine, as this air exits the engine nacelle?” That’s the only way I can make sense of this.

@Penguin: Not only confusing, as (if using your reformulated question) impossible to answer as it depends on the design and the airspeed.

I would think that this would be heavily (totally) dependent on the aircraft and engine under consideration. They all use outside air in some manner to cool the engine(s), but how (fast) it exits the nacelle is specific to each particular installation - whether it flows through based solely on the aircraft's forward motion or if it's fan propelled; how many twists & turns it may take on its way out; how many other components it might cool before/after hitting the block & cylinders; etc.

I voted to close this question - it is too broad. Each cowl of every model of engine and aircraft will be different dependent on opening size, exit size, altitude, speed, and drag within the engine compartment, not to mention the availability of cowl flaps and their position setting.

I am beginning to think that the estimation of such velocities is not widely known and is in the purview of a specialized investigation for each aircraft case. However, it also cannot be said that these velocities cannot be estimated, which would be necessary during conceptual design (apart of which I am at the moment). I have looked through the matter over the past few days and have found a conclusive method to estimate this velocity. For future reference of others, kindly reopen the question in order that I may provide my finding as an answer. Thank you.

I have reopened it. please post what you have found out :)

Thank you for reopening. Give me some time. I am preparing the answer with some calculations. Will post over the next few days.

There are cases, as in the P-51 Mustang, where the cooling air adds to the THRUST of the aircraft as it is heated and expelled out the back of the air vent, much like a jet.  This actually added a few knots its speed.
There is always an issue of cooling as one scales up a fuel burning engine.  Towards the end of their time, piston engines for aircraft had grown exponentially in power in the race for performance.  This lead to major issues in cooling of engines such as the "corn cob" R-4360 Wasp Major.  Turbo or Super chargers also unavoidably raise the temperature of incoming air by compressing it.
So to estimate the velocity of the cooling airstream, one might begin a study of the principles of a jet engine.  Since no mass is added to the cooling stream and there is no compressor (as in a turbo jet, although prop wash may give some effect) the only considerations are ram pressure and thermal expansion of the cooling air.  Forward speed of the aircraft will be a major factor.
Study of the P-51 design may be a good place to start.


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Why is the gradient line for climb in a one-engine-inoperative condition during take off under CS-25 described in percentage?
Appeciate an explanation in terms of definition of climb in terms of gradient & an explanation of the reasons behind each percentage condition for the different air craft types: twin engine, three engine, four engine, maybe propeller as well?

Gradient as percentage is defined as the ratio between vertical displacement and horizontal displacement. For example, a 5% climb gradient means than for every 1 000 meter of horizontal travel, the aircraft climbs 50 meters. If you want to convert it to an angle you have to use the inverse tangent:
$\textrm{atan}(0.05) \approx 0.05 \ \text{rad} \approx 2.86 ^\circ $
In take-off configuration with the landing gear retracted, the steady gradient of climb for one-engine-inoperative (OEI) is not less than:

2.4 % for two-engined aeroplanes,
2.7 % for three-engined aeroplanes and
3.0 % for four-engined aeroplanes,

(source EASA CS-25, Amendment 20, CS 25.121)
The reason for the difference is that two-engined aircraft loose 50% of their thrust, while four-engined aircraft loose only 25% of their thrust in a OEI situation.
In other configurations there are similar differences.


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How can engine positioning affect flutter tendency?
I understand that moving the engines towards the wingtips would increase flutter tendency, but are there any other positioning options which could affect flutter?
How about moving the engine position forward, or backward on the wing. What about the twist of the wings, & the set angle of attack on the ground. Do these have to be considered when positioning an engine on the wings of the airplane?
Also, what about rear mounted engines. What should be considered here to reduce flutter tendencies of the stabilizers?

To better understand what flutter is, see the structure as a mass-spring system like a spring pendulum that has a damper attached to it. The aerodynamic forces both make the spring stiffer and dampen the movement. Crucially, they will act with a delay to the motion of the spring pendulum. Since we are dealing with oscillatory phenomena here, this delay causes a phase shift. The amount of damping increases with dynamic pressure and the phase shift angle decreases with flight speed.
Things start to get interesting when the phase shift angle approaches π. Now the damper will not work against the movement of the spring pendulum but will support it. Instead of reducing the amplitude of the movement, it will increase it - damping will become negative*. Flutter happens.
There are several ways to shift that point outside of the flight envelope:

Reduce the mass or inertia of the mass-spring system: Now the eigenfrequency of the system will be higher, so the phase shift will approach π at a higher speed. Moving the engines closer to the fuselage will increase the bending eigenfrequency of the wing, so any flutter that involves wing bending will be shifted to higher speeds. A typical case would be the flutter resulting from the coupling of the fast-period mode with the wing bending mode in swept-back flying wings.

Make the spring stiffer: This again raises the eigenfrequency of the spring, so a stiffer structure will shift the flutter speed up. This is no panacea, however: When the Rockwell Commander 112 was developed, flight test revealed flutter of the horizontal tail. A professor from Kansas recommended stiffening of the tail spar and was so convinced that this would cure the flutter that he joined the next test flight, only to crash with the aircraft. As it turned out, the stiffening shifted the flutter speed up such that the forces involved broke the spar while flutter was non-destructive with the softer spar at a lower speed (like in this video).

Change the Eigenmode of the spring: If the motion of the bending wing will at the same time add a torsion moment which twists the wing, resulting in a lower local angle of attack and, consequently, lower lift on the up-moving part of the cycle and vice versa, the aerodynamic damping can be increased. By shifting the engines forward, the engine mass will introduce such a torsion moment into the wing. Of course, when placed further outboard, the engine mass will cause a larger torsion angle with the same torsion moment, so shifting the engines out towards the wingtips makes this more effective.

Shift the resonance speed below minimum flight speed: This might sound silly, but works sometimes: When flying fast enough, the phase shift can be small enough to turn aerodynamic damping back into the positive region, and the plane will not flutter with that particular flutter mode. But it might still flutter at a harmonic …

Generally, to achieve positive damping, the center of gravity of a part should be ahead of its elastic axis. In the case of control surfaces, their center of gravity should be in or ahead of their hinge line. That is the reason for balancing masses!

* to be precise: As long as damping retards the motion, the damping term is mathematically negative and turns positive when it starts to support it.

pi is in radians right?

@Abdullah pi is just a value, but the phase shift angle is in radians, yes.

This is a very interesting question, with a very complicated answer!
The problem of Aero-Elasticity is not easy or simple to solve. The reason is, that the equations (or better the system of equations) which describe it are not simple (in this case the non-linearity of the aerodynamic forces create most of the analytical problems). Flutter can be assessed with an aero-elastic simulation.
The following graphic (from EPFL) illustrates the system of equations.

As you can see there are  roughly speaking three things (and their interaction) a designer needs to keep in mind: Dynamics, Fluid-Mechanics, and Structural Mechanics. Usually this is done using simplified versions of the airplane like the following (taken from DLR).

Based on this simplified representations of the air-plane trade-studies can be done to collect a better understanding.
The TL;DR answer to your question is: Yes, all the things you mentioned need to be taken into account to design an air-frame.
When designing an airplane the position of the engine does matter. But it is not only the position but also the mass and the elastic properties of the air-frame. On top of this the aerodynamic forces need to be taken into account as well.
There are no simple recipes to reduce flutter because most of them only work for a limited set of boundary conditions.
A very (vague) basic one could be: Place engines so that manoeuvre loads will result in a small (as small as possible) elastic deformation (thereby reducing the amount of potential energy which is stored in the airframe).
The other recipe is to look what others have done. Given you are planing to use the aeroplane in a similar way other manufacturers offer aeroplanes and given you are using similar materials and manufacturing techniques: you could use existing designs as starting points for your trade-study.

Thanks. But when considering designing an aircraft, we should be able to judge where the positons of the eingine ought to be prior to starting off with engine sizing, & modelling, including the aircraft frame modelling. It is at this juncture that I ask this question, so that we would have a clear heading for the modelling later. How should we go about positioning the engine to avoid flutter?

I am afraid this is not how design works in a complex system. There has to be some iterative work in order to tune your design. Since I do not know what kind of aero plane you are envisioning (like size, mission, cost etc.) it’s hard to suggest a starting point.

No, moving the engines towards the wingtips would not increase flutter tendency. Flutter can be pictured as the tendency of the airfoil to twist itself perpendicular to the airflow. The wing structure is elastic, and the elastic torsion box springs the wing back again - flutter is just like any underdamped spring-mass-damper system that can start to oscillate.
In general, the measures that can be taken against flutter are:

Reduce the twisting force: reduces the amplitude of the oscillation
Increase the torsion stiffness. Make it stiff enough and there is hardly any oscillation
Increase damping forces. Reduces the number of overshoots and the amplitude. This one is not easy to do in a wing construction.

Increasing the mass of the mass-spring-damper system increases overshoot amplitude, increases the time period of the oscillation, and increases the number of overshoots.
So the practical measures that can be taken against flutter are:

Mount an engine near the wing tip, forward of the aeroelastic axis, this reduces the backward twisting force of the airstream. The closer to the wing tip, the better it does its job as flutter force reducer and wing bend reliever. But too close to the tip and the landing jolt will be excessive: there is an optimum.
Mount the engine at the optimum distance forward of the aeroelastic axis, too far forward and the construction for increasing torque stiffness will become too heavy. Too far back and it does not do its job well.

The design twist of the wing is computed at cruise conditions, set angle of attack on the ground is not really critical if the plane can rotate back far enough.
Mounting the engine at the back of the fuselage in pods is basically not done anymore for larger aircraft, that question has been answered a couple of times on this site.

Ok, I can see your point, that adding the weight (engines) to the wingtips would in fact counter the bending moments by lift, which would stabilize the wing during flight; however, during flight, I would see the engine at the wingtips as a mass at the end of a cantilever spring => this is a recipe for flutter in my opinion. I've also read that translating the engines to the wingtips increases the aeroelastic effect, which adds weight to my argument here I suppose. But what's your opinion on the matter?

Yes indeed, the wings flap up and down more if the engine is positioned closer to the wing tip. But flutter is not the flapping, it is the twisting backwards of the wingtip. The wing bends upwards in flight, the closer to the tip the engine is mounted the higher the wing bending relief. Period and amplitude of flapping oscillations would increase, however this motion has a high aerodynamic damping.

More information in this question

@Koyovis: you are over-simplifying the flutter phenomenon! It is not the torsional mode itself which causes flutter but the crossing of two eigen-modes (often first bending and first torsion). Increasing stiffness (torsional and beding) might not always be the right way to go. The solution is to separate the two modes as much as possible, within the flight-evelope.

@Koyovis: flutter is the coupling of a torsional and a bending mode of the wing. The aero-elastic phenomenon which twists a wing backwards is called diversion and can be modelled and predicted a lot easier because it does not feature the unsteadiness of flutter.


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What is the link OR difference between change in momentum & pressure difference?
I ask in regards to the the thrust formula, which includes change in momentum, as well as difference in pressure between inlet & outlet of the engine to calculate generated thrust; this implies that momentum change & pressure difference are clearly two (2) different phenomenons.
But in reaction turbine blade cascades, we can observe a pressure distribution similar to that of an aircraft wing, producing lift which causes the turbine to spin, & a load component acting on the turbine bearings. Also, the inlet & outlet momentum change is seen to cause the turbine to spin. I would expect that a turbine's rotation can be explained by either of the explanations but not both together, since the pressure distribution over the blade profile is a consequence of the deflection of air by the blade, which also causes the change in momentum of the flow exiting the turbine. So the force acting upon a turbine blade can be calculated either by calculating momentum change, or by calculating pressure difference, but not as a sum of both.
Now, returning to the case of the engine, of course, there isn't a pressure difference vertically to the axis of an engine, so no force in this direction. But the pressure difference in line of the axis is a direct consequence of the acceleration of the flow (momentum change) through the engine:
So why is pressure difference & momentum change summed together to get thrust?

It's mr Bernouilli's signature discovery: total pressure is dynamic pressure (momentum change) plus static pressure. Both static and dynamic pressure are the result of the impact of air molecules upon the wing or turbine blade, which acts as the lifting surface. The main distinction between the two is that static pressure is the result of random velocity vectors averaging zero, while dynamic pressure has an average molecule velocity unequal to zero.
Bernouilli's law can also be written as
$$p_t = p_s + \dot{m} \cdot V$$
With $\dot{m}$ the mass of air that impacts the lifting surface per second, and $V$ its velocity. Since that is not a convenient way to make computations regarding aircraft structures, we usually see it in a format that uses air density and wing area.
There is an analogy with potential and kinetic energy. Both are forms of energy, and one form can be freely exchanged for another one. A falling object transforms $m\cdot \Delta h$ into $\frac {1}{2} \cdot \rho \cdot \Delta V^2$, but the total energy remains constant at any point in time.
The same goes for pressure, which is actually a measure of the energy of air molecules. A jet engine adds energy to the air molecules going through it. A turbine blade can be driven by either the dynamic pressure, or by the difference in static pressure before and behind the blade - there are different design blades according to the proportion of static & impulse. The exhaust shape of the engine can leave the outflowing air with a higher static pressure than surrounding air, or expand the higher static pressure completely into dynamic pressure.
Static pressure and impulse change are summed together because they are two forms of exactly the same entity: the energy added to the air stream.

so the reaction of a turbine blade is the total of the pressure difference across the profile AND the momentum change across the blade?

Yes, accounting for efficiency, thermodynamic losses, friction etc.


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How do you design a NACA intake?
What are the construction guidelines to geometrically build a NACA intake (submerged intake) for aircrafts?
The relationships between the aerodynamic performance parameters (ram recovery, drag, and mass flow rate) with the geometric parameters (width of the intake, height, length of ramp, ramp angle, inlet radius and angle, divergent wall angles, divergent wall curve radius, etc...) would provide a guide to begin the construction process for a base model, which could then be refined with further investigation for a specific aircraft.
There are even tools to construct the curve of the NACA intakes. However, the background of the tool as its aerodynamic justification isn't available or at least not found.
I would therefore greatly appreciate any material on the correct geometric construction of NACA intakes.

NACA itself published quite a few papers on their submerged duct design. As a starter have a look at:

Mossman, Emmet A., et al. NACA-RM-A7I30 "An experimental investigation of the design variables for NACA submerged duct entrances", 08. January 1948
Frick, Charles W., et al. NACA-ACR-5I20 "An Experimental Investigation of NACA Submerged-Duct Entrances", 01. October 1945

The NASA Technical Reports Server as well as the UK archive of digitised NACA Reports offer a lot more material for further investigation.


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What are some ways to decrease turbulence?
I know there are ways turbulence can be reduced, but what are these ways that keep the ride comforting and also with better stability.

Turbulence can't be reduced.  It can be avoided and the effect of this natural phenomenon can also be minimized.  The common ways of doing this are avoiding areas of turbulence by altering your route or altitude, and by slowing down your airspeed.

The only turbulence you can reduce is the one created by the aircraft (wake trubulence) and it is done by aircraft's design. Otherwise you can reduce effects of turbulence when flying into. You may edit your question to clarify you are asking for reducing effects of turbulences and not turbulences themselves

There is no way to really decrease turbulence per-se since its a factor of the air around you. There are ways to mitigate its effects and some are.

Climb: In some cases the air higher up is less turbulent or you can climb over the weather system causing the turbulent air. In this case climbing to a higher altitude may help
Divert: Similar to the climb situation you can divert around the turbulent air if you know where its occurring.
Timing: This is the one you may have the least control over if you are talking about commercial flying and its not always true but generally speaking the winds and thermal activity tend to be calmer in the early morning and evening. Flying around these times may allow you to find smoother air.
Size: Again this one is hard to control but there is some truth to the fact that big planes will be less effected by turbulent air than small planes. By this I mean that a 777 is going to bump around less than a Piper Cherokee. But it holds more or less true across the board.

The 787 Gust Suppression System should probably get a mention.

I used to fly Fairchild/Fokker F-27s. Fokker believed in, designed, and built rigid wings. The consensus at the time was that this resulted in a slightly rougher ride, a belief that I subscribe to.

Sometimes descending (rather than climbing) gives a better ride.  NOAA has a great turbulence modelling tool which includes a "side view" of the turbulence along a route and can be used to forecast the best altitude (for turbulence avoidance) to fly at.

See also "Valse des Ailerons" - Airbus descriptions give purpose as "turbulence alleviation". Video. This may not be the kind of (more severe?) turbulence intended in the question. Skybrary

You're not going to reduce the actual turbulence, but you can smooth out the ride a little bit.  Slowing down certainly helps.  If you are doing 230KIAS and you hit some bumps it is going to hurt more than at 180KIAS.  The PC12 is notorious for this in my experience.  I found in the Navajo that dropping the gear made the airplane a little more stable in rough weather in the mountains.
I heard an old-timer tell me that pushing the props forward helped because the props work like a gyroscope and remain "rigid in space."  I tried it on a particularly rough day where I was making a round trip in SE Alaska in a PA32 and noticed no difference, maybe someone with a physics background could chime in and explain how the guy is right, but in practice, it was just as rough on the way over as on the way back.


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Do vertical stabilizers create vortices like wingtips do?
On a wing, a wing tip vortex is unavoidable no matter what configuration you use, but does a vertical stabilizer form a wing tip vortex? I noticed this picture and saw two vortices coming off of the vertical stabilizers, so was this a result of moving the rudder or does the vertical stabilizer generate lift just like an aircraft wing?

Source

I resized the image for you, it helps to make the question clearer

@Pondlife How did you resize the photo?

I saved it to my desktop, opened it in an editor (Paint, in this case) resized it and saved it again.

Well you're somehow contradicting yourself. If you can't prevent the formation of a vortex on a wing, then any control surface being a wing, you can experience a vortex too. To be accurate: you cannot prevent a vortex when creating lift with a finite length wing.

Good explanation of how the vertical stabilizers are used

In general, the vertical stabilizer and rudder create a symmetric airfoil. As there is no pressure difference between the two surfaces of the vertical stabilizer, vortices doesn't form, unlike the wing, which is designed to produce lift at normal angles of attack (it is usually cambered, producing lift at zero angle of attack).
You're correct that the vortices are generated due to the movement of rudder. Usually, vertical stabilizer doesn't create vortices unless rudder is deflected.
Note: Though I can't be sure, I think the F-22 is braking (the F-22 does not have a dedicated surface for braking like F-15 and uses the normal flight control surfaces). A similar thing happens in the following figure

Source: f-16.net
Only one F-22 (second from top) is using its control surfaces, probably for braking, resulting in vortices from the vertical stabilizer.


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What does YF stand for?
I know that F=fighter, A=attack, F/A=fighter/attack, B=bomber. I can not find any information on what YF stands for. This type of aircraft is usually an experimental aircraft, but why do modern aircraft use X instead of YF?

Modern aircraft?

@egid guess the YF-22 and YF-23 weren't modern aircraft :)

Well I suppose the most modern bits will always be in aircraft that have an "X" (Experimental) designation until they get incorporated into a prototype that will eventually become a line aircraft. (Though somehow I don't think anyone would call the X-1 "modern" anymore…)

The 'Y' in YF stands for prototype according to the Tri-Service aircraft designation system. The 'F' stands for fighter, so YF stands for prototype-Fighter. These aircraft are operated by the US Department of Defense (USAF or USN).
For example, YF-22 is the prototype (technology demonstrator) version of the F-22 Raptor.
The 'X' series is the name given for strictly experimental aircraft; i.e. they are not prototypes.

Why X-32 and X-35, then, when developing the JSF?

@egid they were strictly experimentals, technology demonstrators, not actual prototypes. Mind I've not been able to find a source for the actual percentage of parts commonality, but that should be out there somewhere.

According to "X-15 Extending the Frontiers of Flight" by D. R. Jenkins the X-series was created by NACA as they had no dedicated experimental aircraft program in the 1940s. All research, experiments and tests had to piggyback on other aircraft often unsuited for the tests being performed.
The X-series was started in order to have a designation for aircraft which were never intended to progress into the prototype stage. While some may resemble the subsequent prototypes, they don't represent/contain the full technology stack or a complete set of systems as far as I know.

My best friend was CFTE (Chief Flight Test Engineer) for both the X-35 (Joint Strike Fighter) and the YF-22 Raptor. As well as the SR-71 and the U-2 spy plane.
(I don't do aviation - I'm a Securities attorney)
Both the Y and X in other jets are designated prior to the Jets' first, landing the DOD contract and Second, going into production.
The X-35 won the contract for Lockheed. It's now the F-35. In three different variants. Carrier-bssed (Navy), Ground-based (Air Force), and STOVL (Short Take-Off Vertical Landing).(Marines).
The X was the experimental version of all 3. For example,the Carrier version is now designated as the F-35C (Fighter- Carrier)


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What are static discharges for on an airliner?

I have never noticed these on any boeing planes before, so why are these on airbus a320 series and not on the boeing 737 and also what are they for?

They are static dischargers. See http://aviation.stackexchange.com/questions/2720/what-are-these-things-hanging-off-the-trailing-edge-of-the-wing

This meta question explains how to upload screenshots, it should help you

I think the more interesting thing you got a good picture of in that shot is the nice yellow anchor point for use in over-wing evacuations (see here)

It is difficult to tell by the picture, but they look like static wicks. See this question.

Those are static wicks to dissipate electrical charge which can build up doe to atmospheric friction on he airframe.